Laser diodes are inexpensive and efficient sources of coherent light at high power density and spectral brightness. They are widely used in electro-optical devices ranging from CD players to concrete-cutting industrial lasers. In industrial lasers, laser diodes are frequently used as a pump source for rare earth doped fibers or rods. Laser diodes are also widespread in optical fiber amplifiers, where they are used to pump erbium doped optical fibers.
Reliability is one of the most important parameters of laser diodes. Many factors impact reliability. For example, chip construction, packaging, and heat sinking of laser diode chips can be big factors. When the laser diode chips and packages are properly constructed, the life expectancy of the laser diodes can reach hundreds of thousands of hours.
A common location of failure of a laser diode is its light-emitting end facet, where light intensity is extremely high. Even a minute contamination of the facet, or a minor defect of its crystalline structure, can lead to a catastrophic thermal runaway, forming a melted area or a micro-explosion at the facet, degrading or even completely disabling the laser diode.
The importance of the end facet protection has long been recognized. Tihanyj et al. in U.S. Pat. No. 4,656,638 disclose a method to deposit a passivation layer on laser diode end faces. Yamanaka in U.S. Pat. No. 7,065,117 also discloses a laser diode including a protection layer on its end facet, to strengthen the end facet and reduce a possibility of damage.
Non-radiative electric currents generated in vicinity of laser diode end facets can generate heat, which causes the end facets degradation. Yamanaka in U.S. Pat. No. 6,678,303 discloses a laser diode, in which recesses are formed in the doped layers above and below the p-n junction, to prevent the non-radiative currents from reaching the end facet. Schmidt et al. in US Patent Application Publication 2008/0273563 disclose a ridge-type laser diode, in which degradation of a front section of a ridge-type laser diode is lessened by providing a trench or gap on the ridge between an active section and the front section. The gap limits the carrier injection into the p-n junction to the active section, preventing the carriers from reaching the end facet, thereby lessening chances of a catastrophic failure of the end facet.
Harder et al. in US Patent Application Publication 2010/0189152 disclose a laser diode, in which the end facet degradation due to non-radiative carriers is lessened by controlling the currents flowing in the laser diode by providing a plurality of current-injecting electrodes along the longitudinal axis of the laser diode, and by regulating the current separately in each electrode to generate a desired longitudinal profile of current injection.
In the laser diodes of the prior art, the light-emitting end facets have been protected by including protective coatings or forming trenches or gaps on ridge structures, or by providing multiple electrodes to create a pre-defined current distribution inside the laser chip. It is a goal of the invention to lessen a rate of degradation of an end facet of a multimode laser diode chip without having to modify or otherwise complicate the structure of the laser diode chip.